'Collapsing rings' on Schottky electron emitters

The electron beam in systems that use a Schottky emitter as the electron source can display periodic fluctuations when the emitter is operated at an extraction voltage that gives a relatively low field strength at the tip. In the past, these fluctuations have been associated with the so-called "collapsing rings" without much further information. In this paper, the tip's geometry changes associated with these beam instabilities are investigated in more detail by recording the evolution of the emission pattern of a Schottky emitter showing 'collapsing rings' for different operating conditions. Scanning electron microscope (SEM) images of different Schottky emitters have been used to support the interpretation. The beam instabilities generally occur with large intervals. It was found, however, that the emitter is changing its tip end geometry continuously and in a repetitive process. Such a cycle starts with a {100} facet at the tip end. This facet decreases in size, and then a large ring-shaped step is formed that changes the tip end geometry into one with a {100} island of tens of nanometers high, on top of a {100} facet. The cycle is finished when the atoms of the island all have been transported away and the facet underneath is fully exposed. The cleaning up of the island was found to be a repetitive process in itself, in which each time the (uppermost) island is reduced in size, and then splits into two islands lying on top of each other. We found that at practical operating conditions, the geometry of the stacked islands is typically asymmetric, and becomes symmetric only for higher temperatures and lower fields. The characteristic beam current fluctuations are associated with island edges moving near or in the area that delivers the electrons for the beam. The relatively constant beam current in between fluctuations is the net result of the two effects: the current density on the facet center increases due to the increasing local field strength with decreasing facet diameter, but the beam divergence between the facet and the extractor also increases, as a consequence of the changing geometry. Although concealed by the relatively constant beam current in between the fluctuations, for a collapsing emitter the important properties as the brightness, energy spread, and virtual source are thus changing continuously. The known remedy against beam instabilities is to increase the extraction voltage, but this is usually a reactive response: the voltage is increased after a fluctuation has occurred. This study suggests that by monitoring the total tip end current and/or the field enhancement factor of the tip, the facet size reduction that heralds an instability can be detected early enough to prevent it from happening.